U.S. patent number 5,357,958 [Application Number 08/033,551] was granted by the patent office on 1994-10-25 for interventional mri system and rf coils therefore.
This patent grant is currently assigned to The Regents of the University of California. Invention is credited to Leon Kaufman.
United States Patent |
5,357,958 |
Kaufman |
October 25, 1994 |
Interventional MRI system and RF coils therefore
Abstract
An MRI RF coil utilizes a helical conductor having a plurality
of turns and a non-uniform pitch between turns along at least a
portion of the axial length of the coil. The non-uniform pitch is
disposed so as to accommodate interventional procedures and/or
interventional apparatus (e.g., stereotaxic frames) when used on a
human patient in conjunction with magnetic resonance imaging. The
non-uniform pitch portion of the coil may comprise a simple gap, an
opening formed by deformation or angular disposition of some or all
of the coil turns and/or by utilizing fringe field effects of the
coil to accomplish at least some of the magnetic resonance imaging
procedure.
Inventors: |
Kaufman; Leon (San Francisco,
CA) |
Assignee: |
The Regents of the University of
California (Oakland, CA)
|
Family
ID: |
21871073 |
Appl.
No.: |
08/033,551 |
Filed: |
March 18, 1993 |
Current U.S.
Class: |
600/410; 324/318;
324/322; 600/13; 600/422 |
Current CPC
Class: |
G01R
33/34053 (20130101); G01R 33/34084 (20130101); G01R
33/3628 (20130101) |
Current International
Class: |
G01R
33/34 (20060101); G01R 33/32 (20060101); G01R
33/36 (20060101); A61B 005/05 () |
Field of
Search: |
;324/318,322,309
;128/653.2,653.5 ;600/13-15 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Cohen; Lee S.
Assistant Examiner: Casler; Brian L.
Attorney, Agent or Firm: Nixon & Vanderhye
Claims
What is claimed is:
1. An interventional MRI system comprising:
a transverse magnet MRI apparatus;
an interventional appliance having predetermined maximum dimensions
and adapted for physical interaction with a portion of a human
anatomy to achieve interventional procedures therewithin during MRI
within said transverse magnet MRI apparatus; and
a solenoidal MRI RF coil located within said transverse magnet MRI
apparatus and having a diameter less than the maximum dimensions of
said appliance further including means to accommodate said
appliance when used on a human patient for coupling MRI RF fields
to or from the human anatomy and thus facilitating said
interventional procedures during MRI.
2. An interventional MRI system as in claim 1 wherein said RF coil
comprises:
a plurality of conductive turns juxta-posed along an axial
dimension with said appliance and the human anatomy being imaged
such that MRI is achieved, at least in part, using a fringe RF
field of said RF coil which fringe field is disposed beyond an
outer boundary of said solenoidal coil.
3. An interventional MRI system as in claim 1 wherein said RF coil
comprises:
a helical conductor of diameter D and axial length L having a
plurality of turns and a non-uniform pitch along at least a portion
of the axial length thereof disposed so as to accommodate said
appliance therewithin.
4. An interventional MRI system as in claim 3 wherein said
non-uniform pitch portion includes a gap of axial length G between
two turns of the coil with remaining turns being uniformly
separated by pitch distance P which is substantially less than
G.
5. An interventional MRI system as in claim 4 wherein said two
turns are conductively connected by an axially-extending conductor
disposed approximately tangent to the circumference of said
coil.
6. An interventional MRI system as in claim 4 wherein said two
turns are conductively connected by a conductor which extends
radially beyond the circumference of said coil and also in an axial
direction to provide a cleared volume surrounding the coil diameter
at said gap area.
7. An interventional MRI system as in claim 3 wherein said
non-uniform pitch portion includes a first group of turns disposed
at an angle+.alpha..degree. with respect to a perpendicular
cross-section of the coil and a second group of turns disposed at
an angle-.beta..degree. with respect to a perpendicular
cross-section of the coil so as to provide an angular opening
.alpha.`+.beta..degree. between said groups of turns.
8. An interventional MRI system as in claim 7 wherein .alpha. and
.beta. are approximately equal.
9. An interventional MRI system as in claim 3 wherein said
non-uniform pitch portion includes axially-extending offset
deformations in said conductor along a circumferential portion of a
subset of said turns thereby providing opened access inside of said
coil.
10. An interventional MRI system as in claim 1 wherein at least one
half of said coil includes conductor turns having selectively
disconnectable joints therein.
11. An MRI RF coil comprising:
a helical conductor of diameter D and axial length L having a
plurality of turns and a non-uniform pitch between turns along at
least a portion of the axial length thereof;
wherein said non-uniform pitch portion includes a gap of axial
length G between two turns of the coil with remaining turns being
uniformly separated by pitch distance P which is substantially less
than G; and
wherein said two turns are conductively connected by a conductor
which extends radially beyond the circumference of said coil and
also in an axial direction to provide a cleared volume surrounding
the coil diameter at said gap area.
12. An MRI RF coil comprising:
a helical conductor of diameter D and axial length L having a
plurality of turns and a non-uniform pitch between turns along at
least a portion of the axial length thereof; and
wherein said non-uniform pitch portion includes a first group of
turns disposed at an angle +.alpha..degree. with respect to a
perpendicular cross-section of the coil and a second group of turns
disposed at an angle -.beta..degree. with respect to a
perpendicular cross-section of the coil so as to provide an angular
opening .alpha..degree.+.beta..degree. between said groups of
turns.
13. An MRI RF coil as in claim 12 wherein .alpha. and .beta. are
approximately equal.
14. An MRI RF coil comprising:
a helical conductor of diameter D and axial length L having a
plurality of turns and a non-uniform pitch between turns along at
least a portion of the axial length thereof; and
wherein said non-uniform pitch portion includes axially-extending
offset deformations in said conductor along a circumferential
portion of a subset of said turns thereby providing opened access
inside of said coil.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the field of magnetic resonance
imaging (MRI) utilizing nuclear magnetic resonance (NMR) phenomena.
The invention particularly relates to interventional MRI systems
and to RF coils especially adapted for interventional procedures
conducted during or in conjunction with MRI.
2. Description of the Prior Art
Commercial MRI systems of many different designs are now readily
available on the market. Some of these designs use large solenoidal
cryogenic superconducting magnets for generation of the static
polarizing field B.sub.o. Since the patient is totally enclosed
within a long tunnel during MRI on such systems, it is difficult to
contemplate significant interventional procedures to be conducted
during or in conjunction with MRI. However, other commercially
available MRI systems use transverse magnet structures in
conjunction with permanent magnets, resistive or superconducting
electromagnets for generating magnetic flux B.sub.o between
opposing transverse magnet poles). With such transverse magnet MRI
systems, open access to the imaging area has become a possibility.
One such four-poster (for return flux yokes) transverse magnet
structure is depicted in related commonly assigned U.S. Pat. No.
4,829,252-Kaufman (the entirety of which is hereby incorporated by
reference). Such systems are commercially available, for example,
under the trademark "ACCESS" from Toshiba. Here, improved access to
the image volume is already provided by forming apertures in the
large transmit RF coil formers otherwise encompassing the image
volume (even though all four sides of the four-poster magnet
structure are left open by the housing).
A later improvement provides flat pancake-like RF transmit coils so
as to leave the image volume totally unobstructed (except for RF
receive coils). These flat pancake-like RF transmit coils are
described, for example, in co-pending commonly assigned U.S. patent
application Ser. No. 08/025,418, filed Mar. 1, 1993, naming
McCarten et al as inventors entitled "RF Coil Providing Reduced
Obstruction Access to Image Volume in Transverse Magnet MRI System"
(the entire content of which is hereby incorporated by reference).
Unfortunately, MRI processes typically require higher Q, more
closely coupled, RF coils for receiving relatively weak NMR RF
signals.
Interventional apparatus and methods for use with transverse magnet
MRI systems have already been described in various contexts. For
example, see commonly assigned issued U.S. Pat. No. 5,155,435 to
Kaufman et al entitled "Method and Apparatus for Performing
Interventional Medical Procedures Using MRI Imaging of
Interventional Device Superimposed with Ghost Patient Image" and
commonly assigned U.S. Pat. No. 5,184,074, issued to Kaufman et al
(allowed U.S. patent application Ser. No. 07/650,215, filed Feb. 4,
1991) entitled "Real Time MRI Imaging Inside Gantry Room."
As will be appreciated by those in the art, solenoidal RF coils
have found considerable application in transverse magnet MRI
systems. In significant part, this is because a transverse MRI
apparatus provides the static polarizing field B.sub.o directly
between two opposed magnet faces which are typically disposed
horizontally and thus the polarizing field B.sub.o is disposed
vertically therebetween. Accordingly, to obtain the necessary
transverse RF magnetic field for effecting NMR phenomena, a
solenoidal RF coil is perhaps a natural choice. At the same time, a
solenoidal coil disposed around the image volume presents a
substantial obstacle to many interventional procedures.
BRIEF SUMMARY OF THE INVENTION
This invention provides a family of RF coils for use in
interventional MRI procedures. Although the RF coils of this
invention are particularly suited for RF receive functions, they
may also be used for RF transmit functions in certain situations.
The RF coils in this invention find particularly favorable use with
transverse "open" design magnets, real time fluoroscopy with "in
the room" display and "ghost" MRI, although the invention is
clearly not limited just to use in these contexts.
There are at least two general classes of interventional procedures
that doctors may wish to perform during or in conjunction with MRI
(e.g., so as to assist. the interventional physician with more
accurate and/or efficacious interventional results). First of all,
during surgery, a needle may be inserted either to deliver
therapeutic agents to a region of the body and/or to obtain a
biopsy from that region. Such processes may also involve cuts in
conjunction with or instead of needle placement. Secondly, during
stereotaxy, an external physical device (e.g., a frame) is
typically affixed to the patient anatomy such that external
landmarks on the device are related to internal features of the
body. Most typically, stereotaxic procedures are performed within
the cranium with stereotaxic frames being literally screwed into
the cranium so as to provide a stable and known frame of reference
for use of various surgical instruments and procedures.
Such interventional procedures have common and specific needs. For
example, both require a relatively unencumbered working field
around an entrance point. In addition, stereotaxy often requires
space all about the circumference of the relevant anatomy
(typically the head) for attachment of this stereotaxic frame.
Prior to this invention, such space requirements were typically
accommodated by simply using a larger diameter RF coil. However,
this leads to a consequent loss in signal-to-noise ratio and thus
in quality of MRI that is available during interventional
procedures. It also often leaves relatively cumbersome access for
the interventional physician.
The RF coils of this invention provide substantially improved
access to the imaged anatomy so as to better accommodate
interventional procedures during and/or in conjunction with MRI.
For example, a helical conductor having a plurality of turns may be
provided with a non-uniform pitch between turns along at least a
portion of the axial coil length. This non-uniform pitch portion
may be disposed so as to accommodate interventional (e.g.,
stereotaxic) appliances therewithin.
The non-uniform pitch portion of the coil may be as simple as a
bridged gap in an otherwise uniform helical coil. The bridge across
such gap may be either at the same or extended radius depending
upon the nature of the desired/needed access. Alternatively, the
non-uniform pitch portion of the coil may be achieved by disposing
a first group of turns at a positive angle with respect to
perpendicular cross-section of the coil and a second group of turns
at a negative angle with respect to the same perpendicular
cross-section thereby providing an angular opening equal to the sum
of such angles between these two groups of turns. Other
deformations of a portion of the helical coils are also proposed to
create an open access area along at least one side of the RF coil
structure. A still further alternative is to use the fringe RF
field of a specially dimensioned coil to achieve, at least in part,
MRI in conjunction with interventional apparatus/procedures that
are conducted entirely beyond the end boundary of the physical coil
structure.
BRIEF DESCRIPTION OF THE DRAWINGS
These as well as other features and advantages of this invention
will be more completely understood and appreciated by careful study
of the following detailed description of presently preferred
exemplary embodiments taken in conjunction with the accompanying
drawings of which:
FIG. 1 is a schematic block diagram of an interventional MRI system
employing a gapped solenoid RF coil of this invention;
FIG. 2 is a more detailed schematic depiction of the gapped
solenoid shown in FIG. 1;
FIG. 3 is a schematic depiction of a modified gapped solenoid
wherein the bridging conductors across the gap are disposed at an
increased radius so as to provide 360.degree. of, larger radius,
circumferential access to the imaged volume;
FIG. 4 is yet another embodiment of the gapped solenoid of this
invention wherein the gap is formed by complimentary angular
disposition of the coil turns so as to create a gap at one side
thereof;
FIG. 5 is yet another embodiment of the gapped solenoid wherein a
gap is created and one side of the solenoid by providing
axially-extending offset deformations in the coil conductors along
a circumferential portion of a subset of the turns;
FIG. 6 is a schematic depiction of yet another embodiment of this
invention utilizing the fringe field of the RF coil for achieving,
at least in part, MRI in conjunction with interventional
procedures; and
FIG. 7 is a schematic depiction of flexible belted helical coils
having circumferential connectors (e.g., see U.S. Pat. No.
4,897,604--Carlson et al) which may be used in the embodiments of
FIGS. 2-6 thereby facilitating installation of the coils into
operative position after the stereotaxy device is already in
place.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
FIG. 1 schematically depicts a transverse magnet: MRI system. The
main polarizing field B.sub.o is created between horizontally
disposed magnet poles 100, 102 with massive ferrous yoke structure
104 being provided for a return flux path. As those in the art will
appreciate, permanent magnets may actually be located at the poles
or elsewhere within the magnetic circuit 104 (as may resistive or
superconducting electromagnets) so as to provide the necessary
polarizing field B.sub.o.
Flat pancake-like gradient coils 106, 108 are typically also
utilized so as to provide gradients in the polarizing field B.sub.o
along the usual orthogonal coordinates X,Y,Z. As noted above, newly
developed flat pancake-like RF coils (for RF transmit functions)
may also be located at the poles of the magnet in conjunction with
the magnetic gradient coils 106, 108. A greatly simplified
schematic showing of the remainder of the system is depicted by RF
circuits 110, gradient coil drivers sequence control computer
subsystem 114, imaging computer subsystem 116 and an operator
console 118. As those in the art will appreciate, depending upon
which MRI sequence has been selected by the operator via console
118, sequence control computer 114 will then effect a predetermined
MRI data acquisition sequence by activating the RF circuits 110 and
gradient coil drivers 112 to generate a suitable sequence of X,Y,Z
magnetic gradient pulses and NMR RF nutation pulses so as to
generate spatially encoded NMR RF signal responses. These NMR RF
signal responses are then received via the RF circuits 110 and
processed in imaging computer 116 so as to provide a visual image
of the spatial distribution of NMR nuclei within the image volume
120 of the system.
As depicted in FIG. 1, the RF receive coil 122 is a gapped solenoid
having helical turns interrupted by a bridged gap to accommodate a
stereotaxy frame 124 affixed to the head of patient 126. As will be
apparent in the context of the above referenced related co-pending
applications and/or issued patents, there are various ways in which
MRI can be advantageously utilized during or in conjunction with
various interventional procedures.
Preferably at least one side of the coil 122 is provided on a
flexible substrate or former with connectors 700 aligned along an
edge of the substrate (see FIG. 7). In this way, the coil
conductors may be temporarily interrupted and flexed open to
facilitate sideways insertion of patient anatomy located on the
distal side of the stereotaxy frame 124 (e.g., the lower face and
neck as shown in FIG. 1). That is, while the proximal half of the
coil 122 might be installed axially over the relevant anatomy
(e.g., the top of a head), it may be impossible to similarly
install the distal half of the coil after the stereotaxy frame is
in place. The use of a flexible coil former that can be
disconnected for radial installation solves this problem. Of
course, both halves of coil 122 can be formed in the same flexible
manner if desired for maximum flexibility in use.
The gapped solenoid 122 is shown in more detail at FIG. 2. As there
shown, the entire RF coil has a length L and a diameter D. The coil
turns are helical in this example with a nominal pitch P. However,
along a central axial length of the coil, there is a gap G, thus
making the pitch of the overall coil non-uniform along its length
L.
In general, the gap G may quite conservatively be made equal to a
radius (D/2) of the coil or less. With some accepted degradation,
wider gaps may be employed. In this example, the coil diameter D
may, for example, be approximately 12 inches while the nominal
pitch P may be approximately 1.2 inches. The coil conductor itself
may be formed from copper strap (e.g., of 0.010 to 0.020 inch thick
by 1/4 to 3/8 inch wide). The coil may typically include three to
four turns on either side of the gap G.
As depicted in FIG. 2, the bridge conductor 200 across the gap G
and the return conductor 202 pass to a matching/tuning circuit 204.
Here the usual series capacitances C.sub.s and parallel
capacitances C.sub.p are used to resonate the coil (e.g., at about
2.7 MHz in the Toshiba ACCESS system) while also matching the coil
impedance to a desired transmission line impedance (e.g., 50 ohms).
As depicted in FIG. 2, the tuned/matched RF connection is then
connected to the RF circuits 110 (typically via a "ground breaker"
to avoid spurious RF currents passing over the outside of coaxial
cable transmission lines). The matching/tuning circuits 204 may
also be remotely located. In this case, a "ground breaker" is
advantageously employed close to the coil 122 so as to avoid
spurious RF currents over the outer conductor of the
interconnecting coaxial transmission line.
The gapped solenoid structure shown in FIG. 3 is similar to that in
FIG. 2 except that the bridge conductor 200' and the return
conductor 202' are extended radially by the distance d thereby
providing 360.degree. access of increased radius to the image
volume 120. This embodiment may be particularly attractive for
accommodating a stereotaxic device. Since the gap G is typically
not made wide enough to accommodate a human head therewithin
transversely, the patient head (or other anatomy) will typically be
inserted or removed along the longitudinal axis of the coil (for at
least the proximal half of the coil).
The embodiment shown in FIG. 4 includes a first group of turns 400
disposed at an angle +.alpha. with respect to a perpendicular
cross-section 402 of the coil. A second group of turns 404 disposed
at an angle -.beta. with respect to the perpendicular cross-section
402. This provides an angular opening of .alpha.+.beta. (e.g.,
angle .phi. as shown in FIG. 4) between the two groups of turns
400, 404. Preferably, .alpha. and .beta. are approximately equal so
as to provide a symmetrical structure. As will be appreciated from
FIG. 4, the half angle of the opening may be on the order of
30.degree. or more so as to provide a relatively wide gap area G on
at least one side of the coil.
FIG. 5 depicts yet another approach for achieving non-uniform pitch
portion of the helical coil. Here, axially extending offset
deformations 500, 502, 504 and 506 are made in the conductors along
a circumferential portion 508 of a subset of the turns. In this
way, a central opening of dimensions G wide and H high can be
formed in one side of the coil.
Other techniques for providing a non-uniform pitch portion of a
helical coil and thus providing access to the image volume should
be apparent in view of the above examples.
FIG. 6 partially depicts a stereotaxic appliance 124 in place on
the human head of patient 126. Now, however, only one, relatively
thin (in the axial dimension), coil 600 is placed immediately
adjacent the working area of interest. Accordingly, such a coil can
be inserted entirely from the free side (e.g., from the top of the
head in the case of the stereotaxic head frame) as depicted in FIG.
6. From the center of the coil 600, with respect to a radius
distance R and an axial dimension Z, the magnitude of the RF field
drops as the cube root of the quantity (R.sup.2 +Z.sup.2). Using
this relation and assuming a 22 centimeter diameter coil, one may
compare relative sensitivity with respect to the maximum
sensitivity that will be achieved at the coil center.
For example, relative to the expected maximum signal sensitivity at
the coil center, coil sensitivity to RF fields located 2
centimeters from the center will be 95%, at 5 centimeters 75% and
at 7 centimeters only 60% of the maximum coil sensitivity. On the
other hand, if one makes a larger coil (e.g., so as to
conventionally accommodate stereotaxy devices and interventional
procedures), the coil sensitivity is reduced drastically even at
the center of the coil. For example, if the coil is made only four
centimeters larger in diameter (e.g., 26 centimeter diameter coil)
so as to accommodate stereotaxic devices, one is left with only 40%
of the sensitivity expected for the smaller (e.g., 22 centimeter
diameter coil)-even at the center location (e.g., within the center
of the image volume). The further reductions when one is displaced
from the center are similarly drastically affected).
Furthermore, even though a 4 centimeter larger diameter coil might
make it possible to perform some interventional procedures, an
uninterrupted uniform pitch helical coil structure necessarily
typically will provide substantial interference with manipulation
of the stereotaxic device or other interventional procedures that
might be desired. Accordingly, the use of fringe fields as shown in
FIG. 6 not only provides at least as much if not more RF receive
coil sensitivity as the conventional larger diameter coil (and thus
provides comparatively enhanced MRI), it also provides
substantially unimpeded access for interventional procedures as
depicted in FIG. 6. Accordingly, this is considered to be a
substantial. improvement.
As mentioned earlier, for at least half of the coil, it may be
desirable to use flexible, belted solenoids that can be "opened" to
facilitate initial coil placement as depicted in FIG. 7. Here,
greater physical flexibility is provided because this portion of
the coil may be broken apart at a connector 700 and flexed so as to
accommodate patient insertion, set up procedures, etc. One such
belted coil might be disposed and interconnected with fringe field
effects as in FIG. 6. One or two such coils may be utilized in the
configuration of FIGS. 2-5 where one or each half of the coil is
flexible and broken by a connector 700. Extra bridging coil
sections may also be connected in the connector 700 to provide
different coil diameters as required. For more details on creating
variably sized, belted, solenoids per se, (e.g., flexible helical
conductors and a flexible coil former which may be separated along
a axially extending circumferential line by suitable connectors at
that point), see related commonly-assigned U.S. Pat. No.
4,897,604-Carlson et al, entitled "Method and Apparatus For
Selective Adjustment of RF Coil Size For Magnetic Resonance
Imaging" (the entire content of which is hereby incorporated by
reference).
While only a few exemplary embodiments have been described in
detail, those skilled in the art will realize that many variations
and modifications may be made in these exemplary embodiments while
yet maintaining many of the novel features and advantages of this
invention. Accordingly, all such variations and modifications are
intended to be included within the scope of the appended
claims.
* * * * *